CA1111936A - Motor protector - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Apparatus for preventing overvoltage and overcurrent damage to direct current motors, and wheel lockout in electrically powered vehicles having dynamic and mechanical brake assemblies includes dynamic braking voltage and current sensing circuits, a first network responsive to detected dynamic braking voltages exceeding a predetermined value, a second network responsive to dynamic braking currents exceeding a predetermined level, and a third network responsive to dynamic braking current exceeding a different predetermined level at the same time that excessive mechanical braking occurs and for providing an output when such conditions coexist for more than a predetermined time period, and an interrupting circuit responsive to outputs from the first, second, and third networks for interrupting dynamic braking and substituting therefor mechanical braking.

Description

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BACKGROUN~ O~ THE'INVENTION

ield of-t:he'Invention:
The present invention relates to motor protection circuits for direct current motors, and more particularly, to such circuits'in which dynamic braking voltages ana dynamic ~raking currents whic~ excee~ predetermined safe levels are pxecluded while at the same time the application o aynamic and mechanical ~raking sLmultaneously for more tha~- a preset time interval is avoided. .
Desc ~ ~io~ ~'~:t~e ~rio~ ~rt:
Direct current motors, and in particular those which are used to power vehicles of various types, such as trains, cars and trucks, and in operating elevators, winches and other . machinery,are subject to severe mechanical damage if improperly operated during those'periods of time whe~ dynamic bxaking is ~pplied. Dur.ng such aynamic braking, when the motor is acting as a generator and i5 absorbing energy from the kinetic energy o~ ~he mo~ing load and dissipating it in resistors, excessive voltages and/or currents may ~e generated~ Throu~h improper use of the motoX control mechanism, these excessive voltages andfor :~ curren~s may reach and exceed le~els at which electric arcs are . drawn ~etween various parts o~ the motor and/or the frame, .~ meltin~ metal and destroying insulation. These electric arcs, commonly known as flashove~, can produce serious damage requiring ¦ ext-nsiv nd costly repairs and lo s of effective operating tim~

` _ 3 _ -The prior art, as exemplified by U.S. Patents No.

2,656,501, 2,926,759, and 2,933,350, is generalLy cognizant of system~ which automatically regulate the application of dynamic and mechanical or electro-pne~matic braking systems so as to achieve a smooth and ef~icient overall braking characteristic . while at the same time preventing damage to system components.
While these systems are genera~ly effective in achieving desired braking characteristics, t~e fla~er problem continues to exist and in many situations excessive volta~es and/or currents are permitted to develop beyond the limits which can be tolex-ated.relate~ problem has also ~een recognized in the prior art with respect to the common or simultaneous actuation of .
dynamic an~ mechanical braking systems for excessively long 15- periads o~ t~me. In connection with rapid transit systems, for.
. instance, under normal ~raking operation the me~ical` brakes ~hould be appl.ied only when the train speed has s~owed to the point that dynamic ~raking will soon become ine~fective. When ~oth mechanical and dynamic ~raking systems are simultaneously applied while the train is tra~elling at relative high speed,;, the wheels can lock and slide resulting in the production of ~lat spots.
. The tenaency to exhibit wheel locking and the damaging ~ ~Iasho~er which can occur by improper application of aynamic ; 2~ ~ braking substantia1 disadvanlages which have long plaguod . ,.

' ~ : ' ;ystems conventionally used in the past.

SUMMARY OF _ ~E:INVENTION

It iSJ therefore, an object of the present invention to detect excessive voltages and currents in electric motors during periods of dynamic braking, and in response thereto to substi-tute mechanical ~raking thereby avoidin~ flashover damage..
Anot~er o~ject of the present invention is to detect, record, and corxect con~itions of simultaneous àpplication of dynamic and mechanical braking systems for more than a pre-determined time interval to prevent damage.. It is a fur~herobject of this invention to detect conditions which exist in high power circuits and to couple such detected informati.on to control circuits ~or damage prevention, while at the same time . . providing total electri~al isolation between the power and con-: . trol circuits.
: ~he invention m~y be ~ummari~ed as apparatus for pre-;:~ : ; . Yenting da~age to an electric motor during dynamic braking which includes a circuit ~or detecting dynamia brake voltage, .
a network responsive to the detected dynamic ~rake voltage for 2 generating an output whenever the voltage exceeds a predeter-:~ ~ mined value, and a circuit responsive to the output of the voltage responsive network for interrupting dynamic braking and substituting mechanical braking thereor whereby damage to the : moto~ by excessive dynamic ~raking voltages may be prevented.
~5 ¦¦ T~e inven n further oontemplates similar circuitry ior .

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detecting and responding to overcurrent conditions which develop during dynamic braking, and a~ditional circuitry for substituting mechanical braking for dynamic braking whenever the two braking systems are simultaneously applied for more 5 than a preset length of time. ~ ~
The invention exhi~its a number of material advantages over the prior art in that flasho~er in DC electric motors caused by either excessive voltages Qr excessive curxents during dyn~mic braking is eliminated, that wheel locXing and attendant damage caused ~y simultaneous application o dynamic and mechanical braking assem~lies is avoided, and that information relating to operating conditions existing in high voltage power circuits is transmitted to low voltage control circuit~ with complete isolation t~ere~etWeen.
Other ohjects and advantages of the present invention ecome apparent rom the ~ollowing description of the pre~erred e~odiment when taken in conjunction with the accom-panying drawing.
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a RIEE~ :DESCRIPTION :t)F THE DRZ~WING .
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The Figure is a schematic diagram of a preferred embodi-; ¦¦ ment of t motor protector according to the present invention, .~ . . ,.
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DESCRIPTION O~ rrHE PREFERRED E~ODIME~T

Referring to the Figure, a motor circuit 10 as might be used, for example, in a rapid transit system, is designed to be operated from a source of high power operating potential, repre-sented by positive terminal 12. Source~12 is typically 600 volts ¦ DC at several hundred amperes, and is returned to ground at ¦ terminal 14. During dynamic braking the motors are disconnected ¦ from the power terminals as shown by the dashed lines diagram-¦ matically connecting the motor circuit 10 to terminals 12 and ¦ 14. The motor circuit 10 includes four traction motor armatures ¦ 16, 18, 20 and 22. When used in a rapid transit system, arma-¦ tures 16 and 18 may be associated with one truck while armatures ¦ 20 and 22 are on the other. Field windin~s 24, 26, 28 and 30 ¦ are associated with armatures 16 through 22, respectively, with the armatures and field windings of each associated pair connect-ed in series, as shown.
Each of the series branches is connected at their re~
spective terminal ends by variable resistors 32 and 34. A simi-lar variable resistor 36, connected in series with a fixed re-¦ slstor 38,interconnects the midpoints of the series legs to¦ complete the circuit 10, as illustrated. Resi5tors 32, 34 and ¦ 36 are adjusta~le resistors which are controlled to govern ¦ dynamic ~raking by a controller (not shown) of any suitable type.
¦ In operation, the controller increases the amount of dynamic ¦ ~r~king by decreasing resistors 32, 34 and 36~ Should t~e con-¦ troller misoperate and cause the resistances of resistors 32, 34 ¦ and 36 to xeach too low a value for the speed at which the motor ¦¦ is opera ing, excessive field current will fl~w which wlll . .

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generate high armature voltages. If the ~oltage levels ~uring lthese intervals exceed a critical value, typically about 2jO00 volts per pair of motors in a 600 volt system, flashover and attendant`damage will result,.
If the controller misoperates at low speeds, the voltage levels ~ill not rise to the flashover point, but excessive cur-rent will flow, possi~ly causing flashover or damaging resis-tors 32 through 38. Also, such misoperation may have the ~esult of excessively slowing wheel rotation while the train is still .
moving forward, possibly locking.the wheels and causing a flat spot ~y sliaing, I,n order to avo~d t~e above deccri~ed damaging condi-.' tions, the present invention detects and monitors d~namic brake voltage and current levels and substitutes mechanical braking for dynamic braking when dangerous levels of.opexation oc¢ur~
Referring again to the Figure, the most direct way to sense overvoltaae would be to~use connections directly across . each pair' of motors 16-18 and 20-22. Since in modern rapid : transit cars, series motors with low field resistances are gen-erally used, and since modern control circuits include variable . resistors 32 and 34 which have.values genexally much less than . resistors 36 and 38, a good indic,ation of output voltage of all of the motors in the system may be ob~ained by connecting a pair o~ leads ~0 and 42 across the series connection of resis-25 i tors 36 3B, as shown. Wh~le t~s connection is preferrea, _ 8 . .
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it should be appreciated that direct connections across motor pairs 16-18 and 20-22 could ~e su~stituted, i~ desired.
, Leads 40 and 42 feed dynamic brake voltage information to an overvoltage monitoring network 44 which has, at an input, a voltage divider consisting of resistors 46, 48 and 50~ Pre-ferably, resistors 46 and 48 are disposed as near as possible to the motors to minimize the length of cable that might carry heavy power currents in the event of malfunction. Diodes 52 and 54 prevent the aevelopment of reverse voltages acrass resistor S0, ana capacitor 56 *ilters out ~igh frequency pick-up. A
~irst neon lamp 58 is connected in series with a second neon lamp 60, the latter ~eing connected in parallel with a resistor 62~ The lamps are'connected across resistor 50 of the input ~oltage divider and are~responsi~e to the,detected or sensed Yolta~e signal. If the voltage across resistor 50 rises to a high enough ~alue to ~reak do~n neon lamp 58, typica~ly 220 Yolts, lamp 58 will maintain only its conducting voltage, with the di~erence appearing across resistor 62 and neon lamp 60.
, This causes l~n~ 60 to ~ire and become illuminated.
Lamp 60 is disposed within optical proximity of a photoresistor 64. Preerably, lamp 60 and photoresistor 64 are mountea in a light shielded assem~ly so that resistor 64 res~onds only to ?ight ~rom lamp 60. As such, lamp 60 and photoresistor 64'~orm an optical coupler w~ich txansfers the light signal '~ 25 representing an o~ervoltage condition while at the same time i ' :
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maintaining total electrical isolation.
When lamp 60 is dark, photoresistor 64 is nearly an open circuit; however, when the lamp becomes illuminated, the l resistance of device 64 qu~ckly drops to a few hundred ohms 5 ¦ thereby deli~ering current to the gate of an SCR 66 from the anode thereof. This causes the SCR 66 to conduct curxent from a source o~ control potential, represented by terminal5 68 and 70 through the electromagnetic coil of a counter A and a series connec~ed resistor 72. Again, using the example of a rapid transit system, control source 68-70 typically may be a 37.S
volt DC source. By connecting the anode-cathode path of SCR 66 in series with the coil of counter A across the control voltage source, each time SCR 66 fires, an additional count will be accummulated so that a permanent record will-be maintained of the num~er of times an over~oltage cQndition has been experienced .
A ~rake rela~ coil BR is connected in series with a nor-~ally closed em~rgency switch 74 and the collector~emitter path of a transistor 76 across source 68-70. Relay coil BR may be o any conventional type such as that used on modern rapid transi ~0 cars and is connected-to the controller and mechanical brake assemblijes, indicated schematically by block 78. When relay ~coil BR is energized, it acts to hold off the mechanical or air ; brakes and permits normal dynamic brake operation. Whèn de-energizea, it applies the mechanical brakes and effectively ~5 removes dynamic braking usuall~ by increasing the resistances of control resistors 32, 34 and 36 to their maximum values in-dependently of the controller that noxmally governs them. Gen-erally, relay BR is excited from source 68-70, which supplies operating potential during the ~raking cycle and is turned off when the throttle is in the off position. Thus, during normal braking operation, relay coil BR will be energized by the coi-lector current of transistor 76, the base current of which is derived from source 68-70 through resistox 80 and the diode sexies network 82.
A diode 84 is connected between the anode of SCR 66 and the junction of resistor 80 and diode network 82. In this manner, when SCR 66 ~ires, the current through resistor 80 is drained o~f, preventing ~ase current from flowing through network 82 to transistor 76 whiGh is thereby turned of~. With transistor 76 off, ~xake relay BR is de-energized whereupon dynamic braking will ~e interrupted and ~echanical brakes substituted 'herefor . Zencr diode 86 will limit the voltage at the collec-tor o tralsistor 76 to a sa~e value.
In operation, during aynamic braking conditions, when-2a ever the potential across resistors 36 and 38 reaches an exces-sive level, the diviaed potential across resistor 50 of network 44 will be suficiently great to cause neon lamp 58 to ~ire.
~iring o~ lamp 58, in turn, causes neon lamp 60 to become energizea there~y proaucing a rapid drop in the resistance of photoresistor 64~ With the photoresistor 64 at a low .

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~ ~l resistance, SCR 66 is ~ired causlng current to be drawn off t]hrough diode 84 so as to xender transistor 76 nonconductive.
Rlelay coil BR thus drops out and substitutes mechanical braking for dynamic braking. As a result, the dangerously high over-voltage conditions which would otherwise cause damaging flash-over axe eliminated ~efore any breakdown occurs. The above se~uence is also recorded by counter A which stores a total count o~ the number of mal~unctions which have resulted in the arop-out of relay BR.
While ~arious different types of sensors and coupling networks may be utilizea in accordance with the present invention the network described a~ove, including the optical isolation provided by lamp 6~ and photoresistor 64, is preferred for its simplicity and total electrical isolation between high and low voltage circuits. In addition, it has been found that the brake relays on moaern rapid transit vehicles operate rapi~ly enough that tne circuitr~ sho~n will reliably prevent voltage ~lashovex and similar damage. For a slow release brake relay ~R, an anticipation circuit consisting of a resistor 86 and a cap~citor sa connected together in series across resistors 46 and 48 can be used. Resistor and capacitor 86 and 88 cooperate to make the network 44 firing-rate-sensitive, i.e., the neon lamp 58 will fire whenever the rate of increase of the dynamic bxake voltage exceeds a predetermined rate and is appxoaching the bxe~kdown potential of lamp 58. As will be appreciated ~ 12 ~

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blelow, similar anticipa~ion circuits 90 and ~2 may be used with the remaining networks in like manner.
The present invention also includes a dynamic brake current detector network g4. Network 94 is similar to network 44 and receives current information from lines ~6 and 42 which are connected across resistor 38~ Resistor 38 preferabl~ is a one ohm resistor so that the voltage measured across the resis-tor equals the current flowing therethrough~ Lines 96 and 42 are fea to a voltage divider consistiny of resistors 98 and 100, and diodes 1~2 and 104 prevent reverse voltages from operating the circu~t. Neon lamp 106 is connected in series with a neon light source 108 having a resistor 110 connected in parallel, with t~e series connected lamps connected across resistor 100 of the input voltage divider.
The light ~rom lamp 108 is optically coupled to photo-resistor 110 ~hich is coupled to the gate of SCR 11~ in a manner s~ilar to network 44~ A counter B records the number of over-current conditions det0c-ted ~y network g4, and a diode 114 is coupled between the anode o~ SCR 112 and the junction of resis-tor 80 and diode network 82 so as to cause relay BR to drop out in an o~ercurrent condition.
Preferably, when the circpit according to the present inventlon is utilized in association with rapid transit systems, component values are selected such that voltage detector network 44 will cause ~raXe coil BR to drop out when the dynamic brake voltage per motor pair exceeds 1900 volts. Overcurrent network 44 is likewise prefera~ly preset to release coil BR when currents in excess of 400 amperes are sensed. Obviously, the component values may be adjusted or preselected so as to respond to any number o various voltage and current levelsr the exact values of Which will be determined in relation to the overall motor ~onfiguration and aesired braking characterist~c~
A third network 116 prevents the simultaneous app~i-cation of dynamic and mechanical braking systems for excessive periods of time. Input resistors 118 and 120 act as a voltage divider, with aiodes 122 and 124 used to prevent reverse voltage operation. Since network 116 operates at normal rather than extreme conditions, lower voltage sensitivity is re~uired and a Zener diode 126 is used rather than a neon ~ulb. A neon light source 128 is connected in series with the Zener diode 126 : across resistox 12Q of the voltage di~ider so as to respond to : input signals. These signals are derived via lines ~6 and 42 . fro~ acroSs resistor 38 and thus represent dynamic brake current.
Pre~erahly, light source 128 will be energized whenever dynamic ~ 20 brake current is approximately 200 amperes or more.
: ~ ~ . Lamp 128 is optically coupled to photoresistox 130 such that when the lamp is energized, resistor 130 drops to a low i vàlue and a voltage sufficient to trigger an SCR 132 through Zener aiode 134 Will be present across resistor 136. This ~5 vo1~age, ever, will b- held off by the open contacts of a ~14-. ' . ~
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¦ pressure switch 138 which is operated ~y the pressure existing ¦ in the mechanical ~rake s~stem. When the mechanical brake pres-¦ sure exceeds a set point, s~itch 138 will ~e closed, and the ¦ potential across resistor 136 ~ill ~e applied to capacitor 14Q
¦ causing the same to char~e through resistor 142. When the ¦ voltage across capacitor 140 reaches a level sufficient to fire ¦ SCR 132, the SCR will ~ecome conductive and will drain cu~rent ¦ through diode 144 thereb~ causing relay B~ to again drop out.
¦ As ~efore, a counter C records these events and maintains a ¦ record thereof for ~uture reference.
¦ The t~me delay provided by capacitor 140 an~ resistor ¦ 142 is necessary in order to allow both dynamic and mechanical ¦ braking systems to "blend" for a ~ew tenths of a second during ¦ normal stopplng, The time delay thus precludes drop out of I5 ¦ coil ~ until a*ter the normal "~lending" interval has transpired ¦ The present invention also provides for rapid dis-¦ ~harge of capacitor 14Q. ~henever a voltage appears across ¦ resistor 136, i,e., ~uring t~ose ~ntervals when dynamic brake cu~rent exceeds the preset value, a transistor 146 will be held on ~ ~ase currents throug~ its input resistance network. With trans~stor 146 ont transistor 148, which has its base-emitter junction coupied in parallel with the collector-emitter path of transistor 146, is held open. The collector-emitter-path ~f~
transistor 148 is coupled across capacitor 140. Thus t whenever a potential appears across resistor 136, transistor 14~ will be open and capacitor will be allowed to charge~ When the current drops, however, the voltage across resistor 136 drops, transis-tor 146 becomes open, and txansistor 148 becomes conductive to discharge the capacitor completely. .
A~ter one or more of. the SCR devices 66, 112 and 132 have fired,.they will ~e automatically turned off and the syste~
reset whenever the throttle is returned to the off po.sition, since t~is remo~es the control potential from terminals 68-70.
~rom the foregoing, it can ~e appreciated that the present invention enables the sLmple, yet effective monitoring of excessive voltages and currents occurring during dynamic . ~raking, and automatically subst~tutes mechanic21 braking during such detected fault conditions so as to prevent flashover and its attendant damage. The present invention also monitors the simulta~eous application o~ dynaffiic ~raking and mechanical ~raking s~stems for excessi~e periods of time and, again, efecti~ely re~oves-the dynamic-~raking ~efore a~ma~e occurs.
All o~ t~ese operat~ons are recorded ~y separate counters to assist in evaluating o~erall system performace. The circuiks.
: 20 according to the present invention utilize low maintenanae.
: ~ solid state components and proviae optical couplin~ between hi~h and low power portions of the networks to assure total electrical ¦ isolation.
Inasmuch as the present invention is subject to many : 25 variations, modifications and changes in detail, it is intended ~ . .
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b ¦ that all matter contained in the foregoing description or ¦ shown in the accompanying drawing shall be interpreted as ¦ illustrative and not in a lim.iting sense.

Claims (27)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for preventing damage to an electric motor during dynamic braking comprising:
sensing means for electrically detecting dynamic brake voltage of the electric motor;
means responsive to said detected dynamic brake voltage for generating an output whenever said voltage exceeds a predetermined value; and means responsive to the output of said voltage responsive means for interrupting dynamic braking and substituting mechanical braking therefor whereby damage to the motor by excessive dynamic braking voltages may be pre-vented.

2. Apparatus as recited in Claim 1 further including recording means connected with said interrupting means for recording interruptions of said dynamic braking.

3. Apparatus as recited in Claim 2 wherein said recording means comprises a counter.

4. Apparatus as recited in Claim 1 wherein said voltage responsive means includes anticipation circuit means causing the generation of said output whenever the rate of increase of said voltage is above a predetermined rate and the value thereof is approaching said predetermined value.

5. Apparatus as recited in Claim 1 wherein said output is a light signal; and wherein said voltage responsive means comprises a voltage breakdown device coupled to said sensing means, and a source of light; said light source being connected to said voltage breakdown device to generate said light signal in response to operation of said voltage breakdown device whenever said detected dynamic brake voltage exceeds said predetermined value.

6. Apparatus as recited in Claim 5 wherein said inter-rupting means comprises a light responsive device disposed within optical proximity of said light source, and a seim-conductor switch coupled with said light responsive device for assuming a first conductive state in the absence of light from said source and a second conductive state in the presence of light from said source.

7. Apparatus as recited in Claim 1 further including:
second sensing means for electrically detecting dynamic brake current of the electric motor; and means responsive to said detected dynamic brake current for generating a second output whenever said current exceeds a predetermined level;
said interrupting means being further coupled with said current responsive means and responsive to said second output for interrupting dynamic braking and substituting mechanical braking therefor.

8. Apparatus as recited in Claim 7 further including:
second means responsive to said detected dynamic brake current for generating an electrical signal whenever said current exceeds a second predetermined level;
means responsive to mechanical brake pressure ex-ceeding a preset value for assuming an actuated state; and means coupled to said second current responsive means and said brake pressure responsive means for generating a third output whenever the electrical signal from said second current responsive means and the actuated state of said brake pressure responsive means coexist for more than a preselected length of time;

said interrupting means being further coupled with said third output generating means and being further responsive to said third output whereby simultaneous application of dynamic and mechanical braking beyond said preselected length of time is prevented.

9. Apparatus as recited in Claim 1 further including:
second sensing means for electrically detecting dynamic brake current of the electric motor;
means responsive to said detected dynamic brake current for generating an electrical signal whenever said current exceeds a predetermined level;
means responsive to mechanical brake pressure exceeding a preset value for assuming an actuated state; and means coupled to said current responsive means and said brake pressure responsive means for generating a second output whenever the electrical signal from said current responsive means and the actuated state of said brake pressure responsive means coexist for more than a preselected length of time;
said interrupting means being further coupled with said second output generating means and being further responsive to said second output whereby simultaneous application of dynamic and mechanical braking beyond said preselected length of time is prevented.

10. Apparatus for preventing damage to an electric motor during dynamic braking comprising:
sensing means for electrically detecting dynamic brake current of the electric motor;
means responsive to said detected dynamic brake current for generating an output whenever said current exceeds a predetermined level; and means responsive to the output of said current responsive means for interrupting dynamic braking and sub-stituting mechanical braking therefor whereby damage to the motor by excessive dynamic braking currents may be prevented.

11. Apparatus as recited in Claim 10 further including recording means connected with said interrupting means for recording interruptions of said dynamic braking.

12. Apparatus as recited in Claim 11 wherein said recording means comprises a counter.

13. Apparatus as recited in Claim 10 wherein said current responsive means includes anticipation circuit means causing the generation of said output whenever the rate of increase of said current is above a predetermined rate and the level thereof is approaching said predetermined level.

14. Apparatus as recited in Claim 10 wherein said output is a light signal; and wherein said current responsive means includes a breakdown device coupled to said sensing means, and a source of light; said light source being connected to said breakdown device to generate said light signal in response to operation of said breakdown device whenever said detected dynamic brake current exceeds said predetermined level.

15. Apparatus as recited in Claim 14 wherein said interrupting means comprises a light responsive device disposed within optical proximity of said light source, and a semi-conductor switch coupled with said light responsive device for assuming a first conductive state in the absence of light from said source and a second conductive state in the presence of light from said source.

16. Apparatus as recited in Claim 10 further including:

second means responsive to said detected dynamic brake current for generating an electrical signal whenever said current exceeds a second predetermined level;
means responsive to mechanical brake pressure exceeding a preset value for assuming an actuated state; and means coupled to said current responsive means and said brake pressure responsive means for generating a second output whenever the electrical signal from said current responsive means and the actuated state of said brake pressure responsive means coexist for more than a preselected length of time;
said interrupting means being further coupled with said second output generating means and being further responsive to said second output whereby simultaneous application of dynamic and mechanical braking beyond said preselected length of time is prevented.

17. Apparatus for preventing wheel locking in an electrically powered vehicle having dynamic and mechanical brake assemblies, comprising:
first electrical detector means adapted to be coupled with the vehicle electric motor for providing a first output in response to motor dynamic brake current exceeding a preset level;
second detector means adapted to be coupled to the mechanical brake assembly for providing a second output in response to brake pressure exceeding a preset level; and means connected to said first and second detectors for causing the interruption of dynamic braking when said first and second outputs coexist for greater than a predetermined length of time.

18. Apparatus as recited in Claim 17 further including recording means connected with said interrupting means for recording interruptions of said dynamic braking.

19. Apparatus as recited in Claim 18 wherein said recording means comprises a counter.

20. Apparatus as recited in Claim 17 wherein said voltage responsive means includes anticipation circuit means causing the generation of said output whenever the rate of increase of said current is above a predetermined rate and the level thereof is approaching said predetermined level.

21. Apparatus as recited in Claim 17 wherein said first output is a light signal; and wherein said first electrical detector means includes a breakdown device and a source of light; said light source being connected to said breakdown device to generate said light signal in response to operation of said breakdown device whenever said detected dynamic brake current exceeds said preset level.

22. Apparatus as recited in Claim 21 wherein said interrupting means comprises a light responsive device disposed within optical proximity of said light source, and a semiconductor switch coupled with said light responsive device for assuming a first conductive state in the absence of light from said source and a second conductive state in the presence of light from said source.

23. Apparatus as recited in Claim 22 wherein said second output is a pressure signal; and wherein said inter-rupting means further comprises a pressure responsive switch;
said pressure responsive switch being coupled between said light responsive device and said semiconductor switch for preventing said semiconductor switch from assuming said second conductive state until said brake pressure exceeds said present level.

24. Apparatus for preventing overcurrent and overvoltage damage to a traction motor and for preventing wheel lockout in an electrically powered rapid transit system having a brake relay and a mechanical brake assembly, comprising:
means adapted to be connected to the brake relay for controlling energization thereof;
first sensing means for electrically detecting dynamic brake voltage of the traction motor and generating an output whenever said dynamic brake voltage exceeds a pre-determined voltage value;
first means coupled to said brake relay control means and operatively communicating with said first sensing means for interrupting energization of the brake relay in response to the output of said first sensing means;
second sensing means for electrically detecting dynamic brake current of the traction motor and generating an output whenever said dynamic brake current exceeds a first predetermined current level;
second means coupled to said brake relay control means and operatively communicating with said second sensing means for interrupting energization of the brake relay in response to the output of said second sensing means;
third sensing means for electrically detecting dynamic brake current of the traction motor and generating an output whenever said dynamic brake current exceeds a second predetermined current level;
means adapted to be coupled with the mechanical brake assembly and responsive to mechanical brake pressure exceeding a predetermined value for assuming a particular operative state; and third means coupled to said brake relay control means and said brake pressure responsive means and operatively communicating with said third sensing means for interrupting energization of the brake relay in response to the output of said third sensing means only when said brake pressure responsive means is in said particular operative state;
said first, second and third sensing means being electrically isolated from said first, second and third interrupting means.

25. Apparatus as recited in Claim 24 further including first, second and third counters coupled with said first, second and third interrupting means, respectively, for re-cording interruptions of the energization of the brake relay.

26. Apparatus as recited in Claim 24 wherein the outputs of said first, second and third sensing means comprise light signals; and wherein said first, second and third sensing means are in optical communication with said first, second and third interrupting means, respectively.

27. Apparatus for preventing overvoltage and over-current damage to a traction motor and for preventing wheel locking in a rapid transit system having a brake relay and a mechanical brake assembly, comprising:
first means for electrically detecting dynamic brake voltage of the traction motor and generating a first output whenever said voltage exceeds a predetermined value;

second means for electrically detecting dynamic brake current of the traction motor and generating a second output whenever said current exceeds a first predetermined level;
third means for electrically detecting dynamic brake current of the traction motor and generating a third output whenever said current exceeds a second predetermined level;
means adapted to be coupled to the mechanical brake assembly and responsive to mechanical brake pressure exceeding a predetermined value for assuming a particular operative state; and circuit means coupled to said first, second and third detecting means and said brake pressure responsive means and adapted to be coupled to the brake relay for interrupting energization of the brake relay and for maintaining the brake relay de-energized for the duration of each braking cycle in response to said first output, said second output or the coincidence of said third output and said brake pressure responsive means assuming said particular operative state;
said circuit means being electrically isolated from said first, second and third detecting means.